This invention relates to a process for the preparation of cyclic N,N'-dialkyl-ureas. In another aspect it relates to a chemical intermediate which can be hydrogenated to form a cyclic dialkylurea. In still another aspect it relates to a two-step method of converting N,N'-dialkylureas to cyclic dialkylureas. In a more specific aspect it relates to a method of making N,N'-dimethylpropyleneurea.
Cyclic N,N'-dialkylureas are valuable as solvents for a variety of organic compounds such as polymers and aromatic hydrocarbons and also as intermediates for the preparation of other solvents and pharmaceuticals. The cyclic urea, N,N'-dimethylpropyleneurea (DMPU) is an excellent polar aprotic solvent that can be used in many applications as a replacement for dimethylformamide (DMF) and hexamethylphosphoramide (HMPA), both of which are suspected carcinogens. The commercial use of DMPU has been severely limited, however, because it has been expensive to produce. There is currently a large market for N-methylpyrrolidone as a replacement for solvents that are possibly carcinogenic. DMPU could help fill that demand if a way could be found to make it at a competitive cost.
Over the last half-century several processes have been developed for the production of these cyclic ureas but none have proven to be successful commercially. As early as 1969 BASF published a British patent, GB 1,173,432, describing the preparation of N,N'-dimethylpropyleneurea and other propyleneureas by the hydrogenolysis of alkoxypropyleneurea. It is stated that prior methods of synthesis have not been commercially attractive because they start with 1,3-propanediamine which is difficult to produce. The process disclosed condenses urea or N-substituted ureas with an unsaturated aldehyde, such as acrolein. The resulting alkoxypropyleneurea is then dissolved in methanol and treated with hydrogen at elevated temperature and pressure in the presence of a Raney nickel catalyst. The hydrogenation removes the alkoxy group leaving only hydrogen or alkyl substituents on the ring.
Other methods of making these cyclic ureas appear to fall into two general groups that can be characterized as follows: (a) those starting with a cyclic urea to which alkyl groups are added to the nitrogen atoms, and (b) those starting with 1,3-propanediamine or dialkylpropanediamine.
Included in the first group (a) is the process disclosed in U.S. Pat. No. 4,617,400, Ito et al. (1986) in which propyleneurea is reacted with formaldehyde under hydrogen pressure of 80 bar, at a temperature of 150.degree. C., and in the presence of a hydrogenation catalyst to make DMPU. Also present is a solid acid made from sulfuric acid and aluminum oxide. The hydrogenation catalysts are reduced nickel or carbon-supported platinum or palladium.
In a 1988 article by Dehmlow and Rao, "Phase Transfer Catalytic Preparation of the Dipolar Aprotic Solvents DMI and DMPU", Synthetic Communications, 18(5), pp. 487-94, a process is described in which a cyclic urea is methylated with dimethylsulfate and potassium carbonate and/or sodium hydroxide. The reactions take place in a solvent such as dioxane or toluene and in the presence of a catalyst such as tetrabutylammonium bromide. The cyclic urea starting material can be made from 1,3-diaminopropane and urea.
U.S. Pat. No. 4,925,940, Franz et al. (1990) describes a process which starts with a cyclic urea that is reacted with formaldehyde to substitute methylol groups on the urea nitrogen atoms. The resulting N,N'-dimethylolpropyleneurea is then hydrogenated under hydrogen pressure of 80 bar, at a temperature of 120.degree. C., and in the presence of a catalyst of palladium supported on an inorganic carrier to make DMPU.
Among the processes which start with 1,3-propanediamine is that disclosed in U.S. Pat. No. 4,864,026, Bickert et al. (1989). In this process the 1,3-propanediamine is reacted with urea to make propyleneurea which is then reacted with formaldehyde and formic acid to add methyl groups onto the ring nitrogen atoms. U.S. Pat. No. 4,900,820, Kajimoto et al. (1990) discloses a process in which urea is reacted with dimethylpropanediamine to form the cyclic urea. In this case the methyl groups are already present on the nitrogen atoms of the propanediamine and are carried forward into the cyclic urea. U.S. Pat. No. 4,918,186, Kajimoto et al. (1990) describes a process which also starts with a symmetrical dimethylpropanediamine in a reaction with phosgene in the presence of sodium hydroxide to make the N, N'dimethyl-substituted cyclic urea.
The processes that rely on symmetrical dimethylpropanediamine as the starting material are not economically practicable because this amine is not commercially available. The processes that use propyleneurea as the starting material require the availability of 1,3-propanediamine as a precursor. This material is relatively expensive because it is made by reacting acrylonitrile with ammonia followed by hydrogenation. Very large excesses of ammonia are required in this reaction in order to minimize the formation of iminobispropionitrile. This by-product has a strong tendency to form because the second addition of acrylonitrile to ammonia is considerably faster than the first addition. Also the hydrogenation conditions of the processes disclosed by the '400 and '940 patents are somewhat severe.
While not directed to the formation of cyclic ureas, an article of interest is one by Polievka et al., "Possibilities of Cyanoethylation of Urea", Petrochemica, 12 (5), pp.122-7 (1972). These authors report attempts to cyanoethylate urea with acrylonitrile that resulted in a mixture of cyanoethylated products which could not be isolated further. The reaction was carried out in a solvent of dimethylsulfoxide in the presence of a basic catalyst. Some polymerization of the acrylonitrile took place, but the cyanoethylated products were thought to be predominantly disubstituted ureas conforming to the formula: NC--CH.sub.2 --CH.sub.2 NH--CO--NHCH.sub.2 --CH.sub.2 --CN. Other products were said have mono-, tri-, and possibly some tetra-substitution. No cyclic products were reported.
Another similar article is one by Halmo et al., "Study of the Product of Cyanoethylation of N,N'-dimethylolurea Using NMR Spectroscopy", Petrochemica, 17 (4), pp. 86-92. These authors reported results similar to that above with cyanoethylation taking place chiefly at the site of the alcohol groups with some addition occurring on the nitrogen atoms of the methylolurea.
It is apparent from the above review that numerous routes have been proposed for synthesis of DMPU and other cyclic ureas, and that several decades have elapsed during the search for a viable commercial process. There remains, however, the need for an economical method of production that relies upon available starting materials and reaction conditions that are industrially practicable.